In recent years, a gallium nitride (GaN) based compound semiconductor material which is a nitride based semiconductor has been attracting attention as a semiconductor material for short wavelength light emitting devices. The GaN based compound semiconductor is formed on substrates made of various oxides or group III-V compounds, such as a sapphire single crystal, using a method such as the metal organic chemical vapor deposition method (MOCVD method) and the molecular beam epitaxial method (MBE method).
A common GaN based compound semiconductor light emitting device is configured so that an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer are laminated in this order when a sapphire single crystal is used as a substrate. Since the sapphire substrate is an insulator, the structure of a device using the substrate will generally be a structure, as shown in FIG. 1, where a positive electrode formed on a p-type semiconductor layer and a negative electrode formed on an n-type semiconductor layer are present on the same plane. In such GaN based compound semiconductor light emitting devices, there are two kinds of devices, that is, a face up type device, in which light is emitted from the p-type semiconductor by using a transparent electrode as a positive electrode, and a flip chip type device in which light is emitted from the sapphire substrate by using a highly reflective film made of Ag or the like as a positive electrode.
When forming a transparent electrode on the p-type semiconductor, a metallic transparent electrode made of Ni/Au or the like has been used conventionally. However, in order to improve the light emission efficiency of light emitting devices, a translucent conductive oxide film made of ITO or the like has been developed on an industrial level in recent years and its use has been encouraged.
The external quantum efficiency is used as an indicator for the improvements in the output of such light emitting devices. When this external quantum efficiency is high, it is possible to say that the light emitting device has a high output.
The external quantum efficiency is represented as the product of internal quantum efficiency and light emission efficiency. The internal quantum efficiency refers to the ratio at which the energy of an electric current applied to the device is converted to light. On the other hand, the light emission efficiency refers to the ratio at which light generated inside a semiconductor crystal is emitted to the outside.
There are mainly two ways to improve the light emission efficiency. One is a method to reduce the absorption of emitted light by the electrodes, protective films, and the like formed on the light emitting surface. Another is a method to reduce the reflection loss which occurs in the interfaces between the materials having different refractive indices such as a compound semiconductor, an electrode, and a protective film.
Incidentally, one of the reason the metallic transparent electrodes made of Ni/Au or the like have been replaced by the translucent conductive oxide films made of ITO or the like is that the absorption of emitted light can be reduced by the use of translucent conductive oxide films.
Examples of the methods for reducing the reflection loss that occurs in the interfaces between the materials having different refractive indices include a technique that forms an uneven pattern composed of a convex shape or a concave shape on the light emitting surface. As an example where the method of forming an uneven pattern composed of a convex shape or a concave shape is applied, a light emitting device in which a compound semiconductor itself is subjected to processing for forming uneven patterns composed of a convex shape or a concave shape has been proposed (for example, refer to Patent Document 1).
However, in the light emitting device disclosed in Patent Document 1, since a semiconductor material is processed, a large load is applied to the semiconductor layer, and the semiconductor layer is greatly damaged. For this reason, although the light emission efficiency improves, the internal quantum efficiency reduces, and thus it is impossible to increase the light emission intensity, which is a problem.
A method has been proposed in which an uneven pattern composed of a convex shape or a concave shape is formed on a sapphire substrate instead of a compound semiconductor itself, and by growing a compound semiconductor on the substrate, the uneven pattern composed of a convex shape or a concave shape is formed on the compound semiconductor as a result, which leads to improvements in the light emission efficiency (for example, refer to Patent Document 2).
However, in the light emitting device disclosed in Patent Document 2, since an uneven pattern composed of a convex shape or a concave shape is formed on a sapphire substrate, the growth degree of a buffer layer to be formed thereafter varies. Accordingly, it is difficult to grow a compound semiconductor stably, which is a problem.    [Patent Document 1] Japanese Patent Publication No. 2836687    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-64492